Recently a gardener called to ask me about using pig manure in her garden. I had always heard not to use pig manure because it may contain disease organisms such as E. coli, salmonella, campylobacter, yersinia, cryptosporidium, giardia. I decided to research this further. I found that these pathogens and others survive not only in the manure of pigs, but also in that of other animals.
Yet, one test where swine manure was applied to potatoes revealed that there were no fecal coliform bacteria present on the harvested tubers. This is important from a food safety perspective. The researchers (Volker et al., 2001) suggested that the soil environment did not present ideal conditions for the persistence of fecal coliforms.
Guan and Holley in 2002 point out that "based on actual data plus some data extrapolated from cattle manure environments, holding (pig) manure at 25°C (77°F) for 90 days will render it free from the pathogens Escherichia coli O157:H7, salmonella, campylobacter, yersinia, cryptosporidium, and giardia. Yet, they point out that improper handling can still cause problems: "Root crops such as radishes and carrots as well as leafy vegetables like lettuce, where the edible part touches the soil, present a great risk for potential health problems."
One possible solution to eliminating the disease organisms in manures is composting.
"In the American organic standards and guidelines, composted manure is recommended for use by organic growers. Raw manure may also be used but not within 90 to 120 days before harvest, depending on the type of crop." (Riddle et al., 1999).
"Proper composting of manure can yield safe fertilizer... if a temperature of 55°C (131°F) or greater is maintained for at least 15 days during the composting period, and that during the period the compost is turned at least five times." (Composting Council of Canada, 2002).
However, before using manures ask yourself does the farmer from whom I'm getting this manure compost at hot enough temperatures for long enough periods of time with sufficient aeration (turning) and moisture? "A two- to four-month composting time has been suggested for backyard composts to get rid of E. coli." (Environmental News Network, 1997).
A study in Canada found that cryptosporidium seemed to be more prevalent in swine than in dairy manure (Fleming, 1999) with greater survival in fresh over old manure.
My colleague Ron Becker at Ohio State University offers: "animal manures can be used as fertilizer on vegetable gardens; however, manure should be incorporated into the soil during the fall prior to planting crops the following spring. Applying manures during the growing season is not recommended due to the chance of contaminating produce with disease-causing microorganisms. Hog manure is just as safe as any other if it is composted or in the ground for at least six months prior to harvesting a crop where the edible portion is in contact with the soil."
Wear gloves when working with manure; wash your hands and your produce thoroughly.
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at firstname.lastname@example.org or 887-2252.